1. Field of the Invention
The present invention relates to an X-ray computed tomography (CT) scanner and a method of collecting image data in the X-ray CT scanner.
More particularly, the present invention relates to an X-ray CT scanner providing means for performing beam hardening correction and to a method of collecting image data in the X-ray CT scanner.
2. Description of the Prior Art
An X-ray CT scanner detects X-rays transmitting through an object being examined and computes absorption coefficients of the X-rays inside the object such as a human body. In accordance with the absorption coefficients, the X-ray CT scanner can reconstruct image data of the sliced planes inside the object.
An X-ray source used for the X-ray CT scanner is the polychromatic source which has multispectrum. That is, the X-ray has a certain energy width on an energy spectrum. For example, FIG. 7A shows the energy spectrum of an incident X-ray into a human body. Moreover, FIG. 7B shows an absorption spectrum inside the human body. The symbol .sub..mu. in FIG. 7B stands for an attenuation coefficient. According to the FIG. 7B, when the X-ray passes through the human body, the X-ray which has low energy level (soft X-ray) is more attenuated than the X-ray which has high energy level (hard X-ray). Therefore, as shown in FIG. 7C, the energy spectrum of the transmitted X-ray detected with an X-ray detector has constituted that the high level energy part increases relatively. As a result, the permeated X-ray has hardened (hereinafter this X-ray hardening phenomenon is called beam hardening).
This beam hardening is more remarkable as the permeation length of the X-ray in the human body is longer. Hereinafter, the length is called the X-ray path length. According to the beam hardening, the CT-value in the reconstructed image decreases from the area corresponding to its circumference of the human body to the center area. Consequently, artifacts appear in the reconstructed image.
In order to solve the problem of the artifacts appearance, the beam hardening correction (hereinafter called BHC), as the correction process with the quality of X-ray, has performed. For example, there is the second pass BHC in the BHC.
In the second pass BHC, the process will be described as follows. That is, the area to be reconstructed is set in advance (hereinafter called the field of view; FOV), the FOV including an region to be imaged, such as a sliced plane in the human body (hereinafter the region is called imaged region). The original image data are reconstructed on the basis of the plurality of projection data In the FOV. Next, a plurality of X-ray path length of the bone part and the mineral part, such as water part, in the reconstructed image are computed. A plurality of beam hardening correction data are generated by using the plurality of X-ray path length of the bone part and the mineral part. Then, image data are reconstructed by using the beam hardening correction data and the original image data.
However, the foregoing process of the second BHC, for example, a magnification display of an interest area in the imaged region or zooming display (display of zooming in the interest area in the imaged region) and so on, in case that the imaged region H is larger than the FOV (A) as shown in FIG. 8, the X-ray path length in the direction of the large part is not computed correctly. As a result, it is difficult to perform the second BHC process correctly. Therefore, the artifacts also appear the border between the bone part and the mineral part in the reconstructed image. The artifacts cause a deterioration in the quality of the reconstructed image.